1. Field of the Invention
The present invention relates to an image forming device such as a color laser printer.
2. Description of the Related Art
One type of conventional color laser printer includes a plurality of photosensitive drums. A different color toner image is formed on each of the photosensitive drums. The different-colored toner images are then transferred from the photosensitive drums one on top of the other onto an intermediate transfer member. The superimposed visible images are then transferred together from the intermediate transfer member onto a sheet in order to form a color image on the sheet. Normally, a fixing unit is provided for fixing the color image onto the sheet.
Normally, the fixing unit includes a thermal roller and a pressing roller. When a sheet formed with a color toner image passes between the thermal roller and the pressing roller, the color toner image is thermally fixed onto the sheet.
Toner can undesirably cling to the thermal roller because of image forces exerted by the charge of the toner or Coulomb forces that accumulate at the surface of the thermal roller. This clinging toner can appear as ghost images in the visible color image fixed on the sheet. This problem can be referred to as electrostatic offset. Electrostatic offset can be prevented by adjusting the configuration of the laser printer in accordance with the charge-to-mass ratio and volume resistance of the toner. For example, a high-resistance element connected between the thermal roller and ground can be selected, or the surface of the thermal roller can be processed, in accordance with the charge-to-mass ratio and volume resistance of the toner.